Diffusion pump



1965 N. MILLERON ETAL 3,224,665

DIFFUS ION PUMP Filed Jan. 12, 1962 4 Sheets-Sheet 1 .7 fq I I INVENTORS NORMAN MILLERON BY LEONARD L. LEVENSON Maw- ATTORNEY 1965 N. MILLERON ETAL 3,224,665

DIFFUSION PUMP Filed Jan. 12, 1962 4 Sheets-Sheet 2 INVENTORS NORMAN M/LLERON BYLEONARD L. LEVENSON 74 75 ATTORNEY 1965 N. MILLERON ETAL 3,224,665

DIFFUS ION PUMP 4 Sheets-Sheet 5 Filed Jan. 12, 1962 INVENTORS NORMAN MILLERON BY LEONARD L. LEVENSON ATTORNEY I 1965 N. MILLERON ETAL 3,224,665

DIFFUSION PUMP 4 Sheets-Sheet 4 Filed Jan. 12, 1962 INVENTORS NORMA/V M/LLERON BY LEONARD L. LEVE/VSO/V ATTORNEY United States l atent 3,224,665 DIFFUSION PUMP Norman Milleron, Berkeley, and Leonard L. Levenson,

Livermore, Calii, assignors to the United States of America as represented by the United States Atomic Energy Commission Filed Jan. 12, 1962, Ser. No. 165,971 17 Claims. (Cl. 230-101) This invention relates to fluid vaporizing and superheating elements, and more, particularly, to liquid vaporizing and superheating elements, especially for diffusion I pumps.

This invention provides a means of vaporizing liquids almost instantaneously without erputive boiling, and also a means of collimating and superheating the resultant vapor stream, or of collimating and superheating vapor produced and introduced from another source. For instance it may be used in diffusion pumps where it is desired both to quickly heat and vaporize the pumping fluid in a collima-ted stream and to thereafter further collimate and superheat the vapor as it leaves the diffusion pump jets. The invention is made possible by any heat producing material or means defining a plurality of prefer- A inch width in which individual turns are insulated by glass tape and in which the entire spiral is integral or in combination with diffusion pump structure in such a manner that the entire apparatus may be assembled and used in a simple manner with maximum efficiency.

Diffusion pumps operate to pump large volumes at low pressures on the change of momentum principle. Basically, a stream of pumping vapor is discharged through pumping jets past an opening having access to the chamber to be evacuated. This fairly unidirectional vapor stream strikes and imparts momentum to the gas molecules emerging through the opening from the chamber to be evacuated. Conventionally, a continuous vapor supply is produced by boiling a pool of relatively involatile liquid, usually mercury or a heavy oil. Since boiling is the reservoir is normally caused by heat exchange with a submerged conventional resistance heater or other heater means external to the pump, the boiling is eruptive. The vapors produced in this manner then pass at a pressure of a few millimeters of mercury into or upward along a standpipe which is usually of a cylindrical design. The vapor then is discharged through jets or nozzles in the form of one or more directed vapor streams. These streams flow past the opening to the chamber to be evacuated and impinge on any gas molecules in their path emerging therefrom and knock or carry them to a region where they can be further pumped out of the system by another stage or series of jets of the diffusion pump and eventually by a conventional forepressure pump.

the vaporizing chamber.

While diffusion pumps of conventional design are satisfactory for most purposes, in this, the advent of the space age, there is an ever increasing need for pumps capable of producing and maintaining ultra-high vacuums, i.e., pressures below 10* mm. of Hg. In order to provide these ultra-high vacuums, the highest possible pumping speed is desired with a minimum of countercurrent contamination from the pump back into the chamber being evacuated. For the pumping of light gases such as hydrogen or helium, the Ho factor presently attainable by diffusion pumps has been limited to less than .5; the Ho factor being defined as the ratio of the rate of gas removal by a given diffusion pump to that which would occur if the pump were replaced by a perfect vacuum.

One of the major obstacles in improving pump performances stems from the eruptive boiling of the pumping fluid. For example, the vapor produced is at an uneven or varying pressure. This causes the momentum of the pump vapor particles in the nozzle streams to vary considerably. The lighter and slower vapor particles tend to wander into the chamber being evacuated. They are also more easily deflected into this chamber.

Another problem is that the thermal efiiciency with the use of conventional heating means is low, not only for diffusion pumps, but also for other apparatus requiring a heated liquid or the production of vapor. Part of the heat energy is required just to keep the liquid at the boiling temperature.

One other disadvantage with eruptive boiling is the relatively long time needed to heat the liquid to its boiling point, as well as for the liquid to cool and stop vaporizing. For many uses, for instance in diffusion pumps for controlled thermonuclear research, it is desired to be able to start and to stop the production of vapors in very short time intervals.

A further difficulty arises in eruptive boiling under low pressure when a leak develops, or the vapor under pressure is otherwise suddenly raised to atmosphere. Liquids and solids are flashed with the vapors and in diffusion pumps this may result in the tarring of the jets and the creation of hot spots which will cause fouling of pumping fluids and other pump materials by chemical decomposition or other processes.

In the vaporization of liquids, as in diffusion pumps, superheating and collimation are also sometimes desired. The speed of a diffusion pump is directly related to the momentum of the pumping fluid vapor streams discharged from the jets. Hence, in order to obtain high pumping speeds, it is desirable to be able to superheat the vapor along a restricted passageway so that a unidirectional velocity is imparted thereto.

There has now been discovered an improved liquid vaporizing and fluid superheating element which avoids or overcomes many of the afore-mentioned problems and difliculties of the art. Broadly the invention comprises a structure defining a plurality of elongated void spaces adaptable to the flow of liquid or vapors therethrough, and generally terminating in a parallel direction, or else in diverse directions determined by particular end uses, in combination with heating means. The structure most often will be at least partially composed of a heat conducting metal used in electrical resistance heating and Wettable by the liquid to be vaporized. A simplified form of the invention would comprise a mass of metal having a plurality of small diameter parallel bores the interior surfaces of which are in combination with structure which conducts heat thereto from another source or creates heat by electrical resistance. However, in the event of dielectric heating at least, the material need not be a metal. The various dimensions, power input, throughput and other factors are variables which are all interrelated in the obtaining of satisfactory or optimum results, according to criteria set forth hereinafter.

The element is adaptable to many'uses, but has particular advantages when structurally integrated within a diffusion pump as hereinafter explained in detail. One embodiment of the heating and vaporizing element, preferred in optimization of costs, simplicity, and other factors, comprises a relatively flat and wide spiral wound tape composed of an electrical resistance heating material, e.g., Nichrome metal, which has, preferably, corrugations transverse to the direction of elongation. Successive turns are insulated from one another preferably by an insulating tape, e.g., glass tape, of approximately the same width. Alternately, especially when the metallic tape is not corrugated, an insulating cord may be separately wrapped a few times transversely around the metal tape at intervals along the length thereof, either construction being simple and inexpensive to fabricate. Depending upon the embodiment, the spiral may be secured in combination with various supporting structure, so that when electrical leads affixed to each end of the metallic tape are energized, resistance heating of the metal takes place.

In the operation and use of this preferred embodiment of the vaporizing element, a liquid or fluid initially occupies the spaces between adjacent turns of the metal and/or insulator. As explained hereinafter, when the channels formed between the transverse corrugations and Wrapped insulator have dimensions within certain limitations, and energy is supplied to the resistance heating element, non-eruptive vaporization of the liquid contained therein commences almost immediately and collimated streams of "vapor are discharged in the direction of displacement. At the same time, liquid in an underlying reservoir is not extensively heated, e.g., the temperature of a reservoir pool did not rise above 100 C. while vapor at a temperature of 390 C, was being produced from it. Vapors passed through the aforementioned channels are also collimated and superheated under proper heat regulation, with consequent increase in pressure and velocity. In addition, heating may be discontinued very quickly, frequently within one second if the heating tape is sufliciently thin that very little heat is stored therein.

Additional advantages of the heating element include the absence of heavy tarring within the system utilizing the vapors produced, e.g., diffusion pump jets, when the vapor chamber is suddenly let up to atmosphere. Because of the limited heating region, chemical damage to fluids introduced into the channels, e.g.,- pumping oils, is greatly lessened. In practice it has been found that conversion and utilization of electrical energy applied to liquids is about fifty percent more efficient than with conventional heating units. For example, a two and onehalf inch diameter heater operating in an oil with the formula m-bis (m-phenoxyphenoxy)-benzene produced a 300 C. vapor stream when operating at 15 amperes and 1100 watts. The oil vaporized at the rate of 1 liquid cc./sec. Using the physical constants for this oil, it was calculated that about 80-0 Watts are required to heat and vaporize it at this rate. This size heater is thus about 73% eflicient as used.

As mentioned before, the basic element of the invention is especially useful when combined with diffusion pump structure, where it may be combined both with the structure of the vapor production chamber and the pumping jets. Specifically, in a preferred embodiment, the vaporizing coil as described is rigidly secured within the vaporization chamber with its channels substantially normal to the surface of the liquid contained therein and emersed within the liquid so that it extends above the liquids surface as described and claimed hereinafter. In addition, other smaller spiral heaters of the invention may advantageously be disposed within the pump jet by combination with flange structure, or other means, as discussed hereinafter.

Incorporation and integration of the heating element within the liquid vaporizing chamber enables pumps to be constructed with relatively little height above the liquid level since the danger of eruptive'boiling and flashing upward into the jets and pump outlet pipe is virtually eliminated. In addition, the collimated spray has a narrow angle of divergence, allowing the pumping stages to be closer together and thus again permitting a reduction in the overall height of pumps. Enhanced pumping speeds and volumes, as well as generally lower pressures, are obtained by elimination of the several problems and difficulties pointed out to exist in the prior art. It is now possible, using diffusion pumps incorporating the improved vaporizing means of the invention, to obtain Ho factors for the lighter gases of at least .85 and even approaching 1.00.

Accordingly, an object of the invention is to provide equipment and a method for vaporizing liquids and superheating fluids to produce collimated jets of vapors of increased or high velocity without eruptive boiling. Another object of the invention is to provide large quantities of vapors with substantially unidirectional velocity over a large cross sectional area. A further object of the invention is to provide equipment and a method of superheating and collimating vapors. One other object of the invention is to provide means for heating and vaporizing liquids with increased efficiency, i.e., more efficient conversion of electrical energy into heated vapors. Another object of the invention is to provide means for continuously vaporizing incremental amounts of liquid within a reservoir at a constant rate with only a minimal raise in temperature of the remaining bulk of liquid contained therein. Still another object of the invention is to provide means for vaporizing liquids within a reservoir in which initial vaporization commences and terminates almost immediately upon the energizing or de-energizing of the heat source, respectively.

A still further object of the invention is to provide a diffusion pump incorporating heating means having the foregoing objects in the vaporization of pumping fluids and/ or in the superheating and collimating of pumping vapors to be discharged from the diffusion pump jets. One other object of the invention is to provide a diffusion pump which has a shorter distance between the pumping liquid reservoir and the first stage of the jets. Another object of the invention is to provide a diffusion pump in which several stages of the pumping jets are closer together than possible in older designs.

Still another object of the invention is to provide structure defining narrow passageways through which liquid or vapor flows in combination with heating means, whereby non-eruptive vaporization or else superheating and collimating takes place during heating. A further object of the invention is to provide such a heatingelement fabricated from relatively thin and wide tapes of resistance heating material coiled so that each turn is in spaced relation with its adjacent turns. Another object of the invention is to provide such a heating element in which the turns of resistance heating material are insulated by an insulating material such as glass type or cord. Still another object of the invention is to provide such a spiral wound resistance heat ng element in which the resistance heating material is corrugated in a direction normal to the plane of the coil.

One other object of the invention is to provide a diffusion pump incorporating one or more heating elements in accordance with the foregoing objectives. Another object of the invention is to provide such a diffusion pump in which such a coiled resistance heating unit is rigidly disposed normal to and emersed within the liquid pumping fluid desired to be continuously vaporized and/or within each pumping jet with the channels of the heating element coaxial with the desired direction of flow of the vapor. A final object of the invention is to provide structure in a diffusion pump embodying one or more of such heating units which facilitates fabrication and use of same, whereby all factors such as cost, efliciency, simplicity of fabrication and dismantling, etc., are optimized.

Other objects and advantages of the present invention will become apparent upon consideration of the following description taken with reference to the appended drawings in which:

FIGURE 1 is a perspective partial view of a preferred embodiment of the vaporizing and superheating element of the invention;

FIGURE 2 is a cross sectional perspective view of a portion of the preferred embodiment of the heating element in which the lower portion is emersed in a liquid;

FIGURE 3 is an elevation view of a conventional diffusion pump partially in cross section and partially cutaway to show preferred embodiments of the heating element in combination therewith;

FIGURE 4 is a cross sectional elevation view of the uppermost jet of the diffusion pump of FIGURE 3 showing a preferred embodiment of the heating element in combination therewith;

FIGURE 5 is a partial perspective view of the vaporization chamber of the diffusion pump of FIGURE 3, partially cutaway, showing a preferred embodiment of the heating element in combination therewith;

FIGURE 6 is a fragmentary cross sectional elevation view of the vaporization chamber of the diffusion pump of FIGURE 3; and

FIGURE 7 is another preferred embodiment of the vaporizing and superheating element of the invention.

Referring now to FIGURES l and 2, there is shown a preferred embodiment of the improved resistant vaporizing and superheating element 11 of the invention comprising a spiral wound, relatively thin and wide heating tape 12 having corrugations transverse to the direction of elongation. Each turn of the tape 12 is preferably insulated from adjacent turns by insulating tape 13 of generally similar width and length wound with the heating tape and contacting the extremities of each corrugation, thereby forming void spaces or elongated channels 14 transverse to the plane of the flat coil. When the heating tape 12 is functioning, as by electrical resistance or conduction of heat from other structure not shown, liquids and vapors passing through channels 14 are heated by conduction.

If the element is to be used to vaporize a conducting liquid, to prevent shorting of the various turns within the liquid, it will be necessary to electrically insulate the metallic tape from the liquid. This can be done most effectively by coating the tape with a thin layer of a ceramic or glass.

Referring now particularly to FIGURE 2 for a description of the operation of the preferred vaporizing and heating element of the invention to vaporize liquids, the spiral wound coil is emersed within the upper portion of a quantity of the liquid which is desired to be vaporized, preferably with the elongated channels 14 substantially normal to the liquid surface 18. The width of the coil 11 and the depth it is to be emersed are dependent upon the amount of energy to be supplied to the liquid and the characteristics desired in the vapor. Normally, the heating tape 12 of the element will be any metal which is used in conventional resistance heaters, e.g., Nichrome metal 6 alloy, so that when electrical leads are connected to each end of the elongated tape and energized, the metal heats and the heat in turn is conducted to the liquid within each of the void spaces or elongated channels 14. While the precise theory or mechanism of operation is not entirely understood, it is thought that it is caused at least partially by the fact that the liquid wets the metal, i.e., at the liquid surface where the element protrudes upward out of the liquid, the liquid miscus tends to curve upwardly from the center of each individual void space and climb or extend up the metallic surface by reason of capillary action and surface tension forces in equilibrium with atmospheric pressure, or other forces which are thought or known to bring about such phenomenon.

Because the liquid wets and climbs the metal tape, the extended surface of the liquid tends to be spread thinly over the metal and it is quickly heated to the vaporization point. As more and more vapor is formed by the continuously climbing film of vaporizing liquid, it is forced upward in the direction of displacement. While it is not thought that the dimensions are essentially critical, and the precise optimum variables, e.g., rate of heating, etc., have not been determined for various liquids and metals, the cross sectional diameter has a minimum practical value, dependent on the density of the liquid to be vaporized, since at very short diameters insufficient liquid will be able to easily enter the lower entrance to replace the liquid vaporized.

As mentioned before, the length of the elongated channels is determined by the rate of heating and the amount of superheating and collimating of the vapor that is desired. If, at high power inputs, the element is emersed too deeply, enough heat will be dissipated into the liquid to cause it to boil eruptively. Equally dependent upon the rate of heating, sufficient structure must extend above the surface of the liquid to ensure collimation and superheating and permit the liquid to creep up the walls. If the top of the element is placed sufficiently close to the liquid surface, fast moving liquid drops may be formed in lieu of or in conjunction with the vapor.

At high power inputs, relative to the size of the unit, there will be a high rate of heat increase and heat dissipation in the formation of vapors and/or superheating and collimating. It has been discovered that eruptive boiling, surprisingly, does not occur even at power inputs which normally would cause flashing or eruptive heating with conventional heating units. While the various variables are more critical at these high inputs, no optimization formula is known. However, it is thought that the cross sectional diameter of the channels must probably be limited to shorter lengths because some heating below the surface at high throughput rates is unavoidable and, as vapors form or extend from the upper surface of the liquid, the smaller diameters prevent the formation of large bubbles which would tend to force or flash any liquid thereabove upward and out of the channel before complete vaporization can occur.

In practice all known resistant metals and low vapor pressure fluids may be used. The following are the maximum and minimum practical values or ranges for each known variable which can be utilized in a high throughput element of any diameter designed for incorporation into diffusion pumps: The channel cross sectional diameter should be within the range 1 to M1"; the channel length should be within the range As" to 3"; and the maximum value of the power input should be limited to 5 watts/in. of heating surface.

Where the function of the element is only to serve as a superheating and collimating means for vapors introduced into one end thereof, the same factors are also determinative of the dimensions in order to ensure proper convective and/ or conductive and/ or radiative heating of the vapors and also to insure the collimation at the velocity determined by the throughput rate and the heating rate.

FIGURE 3 depicts generally a conventional three stage diffusion pump 21 in combination with two coil vaporizing elements 22 of the invention incorporated within its lower portion and another embodiment of the invention mounted in the top jet 44.

Referring now to FIGURE 3, the pump 21 has a cylindrical casing 23 with annular flanges 24 and 26 on the upper and lower ends respectively thereof. Bolts 27 through the lower annular flange 26 rigidly connect the casing 23 to a base 28. An annular metal gasket 29 is provided to insure a hermetic seal between the casing 23 and the base 28. The upper annular flange 24 has bolt holes 31 to facilitate attachment of the pump 21 to either a vessel to be evacuated (not shown) or a conventional vacuum trap (also not shown). The pump has access to the aforesaid vessel or trap by means of large opening 32. Pipe outlet 33 leads from the diffusion pump 21 to a conventional fore-pressure and back-up pump (not shown). Water coolingcoils 34 surround the casing 23 of the pump 21 in order to cool the casings inner surface 36. An oil liquid reservoir 37 rests within the lower portion of the pump 21. A standpipe 38 having three pumping stages extends coaxially upward through the casing 23 from within the oil reservoir 37. The lower two stages of the pump 21 are identical, each having holes 39 and skirts 41 to direct vapor streams outside of the standpipe 38. An elongated bolt 42 extends upward from the base 28 through the standpipe 38 to a water cooled cap 43. This bolt 42 holds the standpipe 38 rigidly within the pump 21.

Cap 43, having water cooling coils 45, covers the top jet 44 of the pump 21. This cap 43 extends over the lip of an inner cap '47 to help prevent backstreaming of vapor. Within the inner cap 47 is one embodiment of the invention which may be better understood by referring to FIGURE 4 which shows the details of the upper jet element.

In FIGURE 4, a conical coil 48 is shown mounted between the inner cap 47 and a conical flange 49 on the upper end of the standpipe 38. This conical coil is constructed of insulating tape 51, preferably glass, and corrugated metal tape 52 as hereinbefore described. Both the lowest 53 and the uppermost 54 turns of the coil 48 are insulating tape in order to insulate the coil 48 from the standpipe 38 and the inner cap 47 respectively. Several clips 56, with their upper surfaces insulated, are provided to hold the coil 48 within the inner cap 47 and to space the cap 47 from the conical flange 49. Clips 57 are also provided to further insure that cap 47 is held in place. The bolt 42 to hold the standpipe within the pump is shown partially in this view passing through the inner cap 47. Insulated electric leads 58 are shown passing into cap 47 along bolt 42 and are suitably connected to the upper and lower turns of the metallic tape 52 to provide power to element 48.

Returning to FIGURE 3 and referring also to FIG- URES and 6, two coaxial vaporizing and superheating elements 22 of the invention are shown mounted on the base 28 within the lower portion of the standpipe 38. There are two coils rather than one since, because of the electrical power capacity of the metal tape, more power can be supplied to two smaller coils than one large one and hence more heat can be produced. These coils, about one inch wide, are emersed approximately half-way along their widths into the oil reservoir 37.

As seen more clearly in FIGURE 5 (which does not show the oil reservoir) these elements 22 are of metallic tape 62 and insulating tape 63 as hereinbefore described. Metallic bands 64, preferably stainless steel enclose the coils about their inner and outer peripheries. Rods 66 of an insulating material, preferably glass, are spaced radially along the circumference of each element both on theirtop and bottom surfaces. Screws 67 rigidly connect these rods 66 to flanges 68 on the metallic bands 64. Because there is an unexplained force tending to cause the elements to move upward during operation, the top rods, where they contact the elements, are flattened to ensure a better rigid positioning of the elements.

The exterior turns of each spiral 22 are of the metallic tape 62 and are in contact with the metallic bands 64. Thin electric plate leads 65, for instance the one shown in FIGURE 6, are connected to the metallic bands 64 and pass beneath the elements to the central portion of .the base 28 where they are attached to electrical connectors 69 passing through the base 28. These connectors 69 are insulated from the base 28 by seals 71. I, An electric power supply (not shown) may be attached to the connectors 69 thereby supplying the electrical energy needed to heat the elements 22.

As seen in FIGURE 6, a plurality of clips 72 are attached to the flanges 68 on the metal bands 64 in order to mount the elements 61 rigidly within the pump. Insulating washers 73 space the clips 72 from the base 28. Bolts 74 pass through the clips 72, Washers 73 and also through insulating seals 75 in base 28.

Also shown in FIGURE 5 is a cutaway portion of standpipe 38 showing slots 76 allowing free flow of the liquid into the standpipe 38. In addition, a portion of the bolt 42 is shown passing through the base 28.

For a description of the operation of the pump, reference is made to FIGURE 3. The diffusion pump 21 is first hermetically sealed by means of flange 24 over an opening having access to the chamber to be evacuated. A forepressure pump (not shown) communicating with the diffusion pump 21 by pipe outlet 33 first reduces the pressure in the vacuum chamber and the diffusion pump to a low level. Then electrical energy is supplied to theelement 48 within the top jet 44. Moments later, when the top jet 44 has reached its operating temperature, power is supplied to the two vaporizing elements 22 emersed in the liquid reservoir 37. Vapor is thus produced in the standpipe 38 by the elements 22 in the manner previously described.

As the pressure within the standpipe 38 builds up, a portion of the vapor enters the element 48 in the top jet 44 of the pump where it is further superheated and collimated. The vapor leaves the element 48 in a collimated stream directed toward the casing 23 of the pump 21. This vapor is incident upon gas molecules entering inlet opening 32 from the chamber being evacuated and imparts momentum to them in a direction away from the opening 32. The molecules are then further pumped by the vapor streams issuing from the holes 39 and directed by the skirts 41 of the lower two stages. The gas, now in the lower portion of the pump, may be drawn off through outlet 33 by the conventional pump.

The oil vapor, striking the cooled interior surface 36 of the casing 23, is cooled and condensed. The condensed oil returns to the reservoir 37 where it can be recycled.

If it is desired, the power to the top jet element 48 may be turned off after the pump has reached its operating temperature. This element then would just give the collimating effect of the invention.

Obviously other heater embodiments may be designed utilizing the principles set forth in connection with the operation of the preferred embodiment, which has been at least partially selected because of the ease and cost of fabrication, as well as good performance. Hence, it is not meant to limit the invention to the preferred-embodiment except as indicated in this specification and the appended claims. Any structure may be utilized which is composed of the proper heat conducting or electrical resistance heating materials and defines a plurality of parallel void channels elongated in at least one direction. If collimation is not desired, the elongated channels may be curved or of irregular shape. The entire unit may be integral, as cast or molded metals, or even solid metal blocks having a plurality of apertures worked therein, as by drilling small holes through the width thereof at close intervals to each other. Other 9 expedients may be utilized to adapt the invention to particular usages.

Another embodiment of the invention utilizing metallic tape is shown in FIGURE 7. Referring to FIG- URE 7, it is seen that the vaporizing element 81 is divorced from support structure and as is the preferred embodiment, comprised of a spiral wound coil of metallic tape 82. However, the metallic tape in this embodiment is not corrugated and adjacent turns are in sulated from each other by insulating cords 83 rather than insulating tape. The cords 83 are wrapped around each turn of the tape so that elongated void channels 84 are formed between adjacent turns of the coil 82 and the insulating cords 83.

The operation of this embodiment is in accordance with the principles discussed earlier and substantially as described with respect to the corrugated tape embodiment.

While this invention has been described with respect to several preferred embodiments, it will be apparent to those skilled in the art that changes and modifications may be made without departing from the true spirit of the invention. Therefore, the following claims should provide the sole measure of the scope of the invention.

What is claimed is:

1. A vaporizing and superheating element to be emersed Within a liquid pool comprising a structure having a plurality of continuous curved surfaces defining capillary void channels, said channels being arranged substantially normal to and extending above the surface of said pool, each of said surfaces being composed at least partially of a heat conducting material and means for supplying heat to said material.

2. A vaporizing and superheating element to be emersed within a liquid pool comprising spaced layers of metallic tape, said layers being spaced by insulating material and defining elongated capillary void channels therebetween arranged substantially normal to and extending above the surface of said pool, and means for coupling electrical power to said metallic tape.

3. A vaporizing and superheating element to be emersed within a liquid pool comprising a spiral of contiguous alternate turns of thin corrugated metallic tape and flat insulating tape forming a multiplicity of small compartments, said turns being arranged substantially normal to and extending above the surface of said pool, and means for coupling electrical power to said metallic tape.

4. The vaporizing and superheating element of claim 3 wherein said metallic tape is nichrome metal.

5. The vaporizing and superheating element of claim 3 wherein said insulating tape is glass.

6. The vaporizing and superheating element of claim 3 wherein the width of said tapes are in the range of A; of an inch to 3 inches.

7. The vaporizing and superheating element of claim 3 wherein the cross sectional diameter of the small compartments formed between said corrugated metallic tape and said insulating tape range from V of an inch to A1 of an inch.

8. In a diffusion pump having a working liquid pool, the combination with said diffusion pump of spaced layers of metallic tape emersed within said pool, said layers being spaced by insulating material and defining a multiplicity of small elongated void channels arranged substantially normal to and extending above the surface of said pool, and means for coupling electrical power to said metallic tape.

9. In a diffusion pump having a working liquid pool, the combination with said diffusion pump of a spiral of contiguous alternate turns of thin corrugated metallic tape and fiat insulating tape forming a multiplicity of small compartments, said turns being emersed within said pool with said small compartments arranged sub- 10 stantially normal to and extending above the surface of said pool, and means for coupling electrical power to said metallic tape.

10. In a diffusion pump having a liquid pool to be vaporized and a plurality of serially operating vapor directing jets communicating therewith, the combination with said diffusion pump of a first set of alternating contiguous layers of thin corrugated metallic tape and flat insulating tape emersed substantially normal to and extending above the surface of said liquid pool, and a second set of alternating contiguous layers of thin corrugated metallic tape and fiat insulating tape disposed within at least one of said vapor directing jets, the cor- "rugations in the metallic tape of said second set of layers being oriented substantially parallel to the desired direction of vapor flow.

11. In a diffusion pump having a liquid pool to be vaporized, the combination with said diffusion pump of at least one annular spiral of contiguous alternate turns of thin corrugated metallic tape and insulating tape emersed within said pool, said turns being arranged substantially normal to and extending above the surface of said pool, inner and outer metallic bands peripherally bounding said annular spiral and in electrical contact with said metallic tape, insulating rods radially extending above and below said annular spiral, said rods terminating at and being rigidly connected to said inner and outer metallic bands, and means for coupling electrical power to said tape.

12. A diffusion pump comprising a substantially vertical open-ended cylindrical casing having a gas outlet transversely communicating with the lower portion thereof, means for cooling the interior surface of said casing, a base plate hermetically sealing the lower end of said casing, a liquid pool to be vaporized contained within the lower portion of said cylinder, a standpipe extending coaxially upward through said casing from Within said liquid pool, said standpipe having a plurality of serially operating vapor jets, the uppermost of said jets having a first set of alternate contiguous layers of thin cor u-gated metallic tape and flat insulating tape disposed in the path of the vapor and having the corrugations of said metallic tape oriented substantially parallel to the desired direction of flow of said vapor, at second set of alternate contiguous layers of thin corrugated metallic tape and insulated tape forming a multiplicity of small compartments which are emersed substantially normal to and extending above the surface of said liquid pool, and means for coupling electrical power to said metallic tape in both said vajor jet and said liquid pool.

13. The diffusion pump of claim 11 wherein the metallic tape of said spiral is nichrome metal and the insulating tape of said spiral is glass.

14. The diffusion pump of claim 11 wherein the bands of said spiral are stainless steel and said insulating rods are glass.

15. The diffusion pump of claim 11 wherein said tapes of said spiral are approximately one-inch wide and extend approximately half-way along their width into said liquid pool.

16. The diffusion pump of claim 11 wherein said liquid pool is oil with formula m-bis(phenoxyphenoxy)-benzene.

17. A diffusion pump comprising an outer sealed casing closed at its bottom by a wall and having input and output conduit connections thereto; a nozzle assembly positioned vertically in the casing and open at the lower end thereof; a liquid charge in the casing; and means for boiling the liquid continuously to generate vapor within the nozzle assembly including at least one curvilinear heater member arranged around the axis of the nozzle assembly; said heater member projecting upwardly from the bottom of the casing through the liquid and above the surface thereof; curvilinear wall portions disposed on each side of said heater member, said heater member being corrugated and cooperable with the adjacent wall portions .to form a multiplicity of very small compartments for References Cited by the Examiner the liquid charge, said compartments being very small UNITED STATES PATENTS by having a horizontal cross-sectional diameter of not greater than one-quarter of one inch, the corrugated 1,406,237 5x 2 Fi' a X heater member and wall portions projecting above the 5 2 3 g zs f z bottom Wall of the pump and extending vertlcally partly 3:038:057 6/1962 Bok et a1. 219 384 below and partly above the level of the liquid charge in the pump whereby the vapors from the boiling liquid in said compartments are additionally heated Within the MARK NEWMAN Exammer' confines of said compartments above the level of the 10 JOSEPH I-I. BRANSON, JR., LAURENCE V. EFNER liquid. WARREN E. COLEMAN, Examiners. 

12. A DIFFUSION PUMP COMPRISING A SUBSTANTIALLY VERTICAL OPEN-ENDED CYLINDRICAL CASING HAVING A GAS OUTLET TRANSVERSELY COMMUNICATING WITH THE LOWER PORTION THEREOF, MEANS FOR COOLING THE INTERIOR SURFACE OF SAID CASING, A BASE PLATE HERMETICALLY SEALING THE LOWER END OF SAID CASING, A LIQUID POOL TO BE VAPORIZED CONTAINED WITHIN THE LOWER PORTION OF SAID CYLINDER, A STANDPIPE EXTENDING COAXIAL UPWARD THROUGH SAID CASING FROM WITHIN SAID LIQUID POOL, SAID STANDPIPE HAVING A PLURALITY OF SERIALLY OPERATING VAPOR JETS, THE UPPERMOST OF SAID JETS HAVING A FIRST SET OF ALTERNATE CONTIGUOUS LAYERS OF THIN CORUGATED METALLIC TAPE AND FLAT INSULATING TAPE DISPOSED IN THE PATH OF THE VAPOR AND HAVING THE CORRUGATIONS OF SAID METALLIC TAPE ORIENTED SUBSTANTIALLY PARALLEL TO THE DESIRED DIRECTION OF FLOW OF SAID VAPOR, A SECOND SET OF ALTERNATE CONTIGUOUS LAYERS OF THIN CORRUGATED METALLIC TAPE AND INSULATED TAPE FORMING A MULTIPLICITY OF SMALL COMPARTMENTS WHICH ARE EMERSED SUBSTANTIALLY NORMAL TO AND EXTENDING ABOVE THE SURFACE OF SAID LIQUID POOL, AND MEANS FOR COUPLING ELECTRICAL POWER TO SAID METALLIC TAPE IN BOTH SAID VAPOR JET AND SAID LIQUID POOL. 